1. Field of the Invention
The present invention relates to a white optical pulse source produced by injecting pump pulses into a waveguided nonlinear optical medium to generate white pulses having a flatly broadened spectrum, and applications of the white pulse source to other optical devices.
This application is based on patent application Nos. Hei 9-161603, Hei 9-274593, Hei 10-81176 and Hei 10-81177 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
As shown in FIG. 23, an optical pulse source is comprise by an optical pump pulse source and a waveguided nonlinear optical medium. A pump pulse propagating in the waveguided nonlinear optical medium induces a third order nonlinear optical effect and generates white pulse of a broad bandwidth. The propagation distance from the input end of the waveguided nonlinear optical medium is designated by z such that at the input end z=0 and at the output end z=L.
A reference 1 (a Japanese Patent Application, First Publication, Hei 8-234249, "Coherent white pulse source") disclosed a white pulse source using a single-mode optical fiber as the nonlinear optical medium, and reported a production of white pulses having an excellent spectral flatness over a wide range of wavelengths as illustrated in FIG. 24. Based on a supposition that such white pulses were produced because of the choice of a low dispersion slope of the waveguided nonlinear optical medium, values of the dispersion slope which is defined as the first derivative of chromatic dispersion with respect to wavelength and magnitudes of dispersion are specified. FIG. 24 shows the spectrum of the pump pulse also.
The reference 1 also disclosed that the bandwidth of the white pulse increases by using a waveguided nonlinear optical medium whose chromatic dispersion decreased with propagation distance z.
This effect is also shown in reference 2 (Okuno et. al., "Study about optical fibers for generating supercontinuum pulses with high efficiency", National Convention Record-The Institute of Electronics Information and Communication Engineers, SB-13-6, 1997). As is shown in FIG. 25, by altering the chromatic dispersion at the center wavelength of the pump pulses .lambda.0 from positive at the input end (z=0) to negative at the output end (z=L), while suppressing the dispersion slope to be low, broadband white pulses are generated. The spectrum of the white pulse produced by the white pulse source in reference 2 is shown in FIG. 26.
In reference 3 (Tamura et. al., "Generation of 10 GHz pulse trains at 16 wavelengths by spectrally slicing a high power femtosecond source", Electronics Letters, vol. 32, no. 18, pp. 1691-1693, 1996), white pulses are generated by a device to pump a waveguided nonlinear optical medium with optical gain (rare-earth doped optical fiber). This device is able to generate white pulses even when the power of pump pulses is low or the length of the waveguided nonlinear optical medium is short.
Here, it should be reminded that, for communication or instrumentation purpose, it is necessary to be able to reproduce white pulses that are not only broadband but remain flat, i.e., a constant optical power output over a wide range of wavelengths.
The conventional techniques disclosed in references 1 and 2 specify only the first order term (dispersion slope) in the chromatic dispersion to wavelengths in the waveguided nonlinear optical medium. In reference 3, the results are associated mostly with doping effects in the waveguided nonlinear optical medium, and the manner of chromatic dispersion reduction is only minimally specified. Therefore, at the present time, a white pulse such as the one illustrated by the spectral curve in FIG. 24 that satisfy both requirements of wide band-width and spectral flattening have not been reproduced in practice.
Further, conventional white optical pulse sources have not been able to generate white pulses having low noise over a wide spectral range, because of the lack of knowledge regarding the mechanism of noise in white pulse generation process.
Further, because it has not been possible to generate low-noise white pulses, a pulse source that combines a white pulse source with a wavelength filter to produce low-noise optical pulses could not be realized also.